Maitotoxin, a highly potent marine toxin (minimum lethal dose of 170 ng/kg, i.p., in mice) isolated by Japanese workers from Gambierdiscus toxicus has been shown to activate voltage dependent calcium channels in skeletal and cardiac muscle as well as certain neuronal and endocrine cells. Maitotoxin has been shown to be cardiotoxic. In the present study we have investigated the mechanism of call death in cultured rat hepatocytes. Cell death was monitored by leakage of lactate dehydrogenase (LDH). Maitotoxin induced a time as well as dose dependent cell death with a LD50 of 60-80 pM at 24 h. The cell killing by maitotoxin was completely abolished by omission of calcium from the culture medium. Cell death induced by maitotoxin was increased by increasing calcium from 30 muM to 3000 muM. Maitotoxin markedly increased influx of calcium within minutes as shown by uptake of 45Ca. The uptake of 45Ca was both time and dose-dependent. However, higher concentrations of maitotoxin caused a rapid uptake followed by leakage of 45Ca which paralleled the leakage of LDH. Verapamil, a specific calcium channel blocker, completely blocked the toxic effect and the 45Ca influx of lower concentrations of maitotoxin and partially blocked the toxicity induced by the higher concentrations of maitotoxin. Maitotoxin induced cell death is due to a marked reduction in cell ATP since it caused a time and dose- dependent loss of cell ATP. Loss of cell ATP occurred following rapid calcium flux. Since calcium is known to interfere with the ability of mitochondria to synthesize ATP, we postulate that the cell killing action of maitotoxin is due to the inhibitory effect of calcium on ATP synthesis.